I played a prot warrior from Molten Core through Sunwell as a main, and then as an alt through all of Wrath. My guild transferred servers, and I only transferred my main at the time, a resto shaman.
During T11, we had been having issues with both tanks being flakes as far as attendance goes. So since tanking is a huge passion of mine in this game, I decided I wanted to MT for my guild again starting in Firelands. My choices were: level a warrior from 1 to 85, transfer my old prot warrior to the new server, or take a chance by making a DK since I hadn't played one before.
In the end, I chose to take a risk on being a DK tank, despite all the rumors (Yes, rumors, because I don't experience any of them) of DK tanks suffering spike damage very often, or DK tanks being a "2 button class", and a ton of other absurd things. And to be entirely honest, it was the best choice I've made in this game so far. I have never enjoyed tanking as much as I do now. DK tanking is just so interactive, and being able to do crazy things that other tanks can't do is part of the fun.
Crazy things like tanking heroic Rhyolith for 60+ seconds, by myself, after everyone else had died. Or beating half my guild's DPS on normal Rag (I'm Blood):
http://www.worldoflogs.com/reports/z...?s=3578&e=3972 or doing insane amounts of healing on heroic Baleroc:
http://www.worldoflogs.com/reports/r...?s=8301&e=8633
If you want to have fun, definitely play a DK, especially with the changes coming in 4.3. If you want to be viable for everything, and be bored while doing it, play a prot warrior.